mediastinum (me"de-ah-sti'num)

Of the ancients, any structure serving as a partition, especially those portions of the pleurae that form the lateral boundaries of the m. of modern anatomists (more properly the cavum medias tin i), which is the interpleural space, or median space of the thorax, between the two pleurae, containing all the thoracic viscera except the lungs, anterior m. That portion of the inferior m. which lies in front of the pericardium, inferior m., lower m. That portion of the m. (1st def.) which is situated below the lower margin of the fourth thoracic vertebra; divided into the anterior, middle, and posterior mediastina. m. dorsale. See posterior m. m. pectorale. The interspace in the chest between the two pleural sacs. m. post i cum. See posterior m. m. testis. See corpus Highmori, under corpus, middle m. That portion of the inferior m. which is situated between the anterior and posterior mediastina, and contains the heart, the ascending aorta, the superior vena cava, the bifurcation of the trachea, the pulmonary ve els, and the phrenic nerves. posterior m. That portion of the inferior m. which is situated behind the pericardium and the roots of the lungs, superior m., upper m. That portion of the m. which is situated above the level of the lower margin of the fourth thoracic vertebra. [Lat., mediastinum, from medius, middle.]